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5 <title>LLVM: Frequently Asked Questions</title>
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10 </style>
11</head>
12<body>
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]13
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]14<div class="doc_title">
15 LLVM: Frequently Asked Questions
16</div>
17
18<ol>
19 <li><a href="#license">License</a>
20 <ol>
21 <li>Why are the LLVM source code and the front-end distributed under different
22 licenses?</li>
23 <li>Does the University of Illinois Open Source License really qualify as an
24 "open source" license?</li>
25 <li>Can I modify LLVM source code and redistribute the modified source?</li>
26 <li>Can I modify LLVM source code and redistribute binaries or other tools
27 based on it, without redistributing the source?</li>
28 </ol></li>
29
30 <li><a href="#source">Source code</a>
31 <ol>
32 <li>In what language is LLVM written?</li>
33 <li>How portable is the LLVM source code?</li>
34 </ol></li>
35
36 <li><a href="#build">Build Problems</a>
37 <ol>
38 <li>When I run configure, it finds the wrong C compiler.</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]39 <li>The <tt>configure</tt> script finds the right C compiler, but it uses the
40 LLVM linker from a previous build. What do I do?</li>
41 <li>When creating a dynamic library, I get a strange GLIBC error.</li>
Reid Spencer669ed452007-07-09 08:04:31 +0000[diff] [blame]42 <li>I've updated my source tree from Subversion, and now my build is trying
43 to use a file/directory that doesn't exist.</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]44 <li>I've modified a Makefile in my source tree, but my build tree keeps using
45 the old version. What do I do?</li>
46 <li>I've upgraded to a new version of LLVM, and I get strange build
47 errors.</li>
48 <li>I've built LLVM and am testing it, but the tests freeze.</li>
49 <li>Why do test results differ when I perform different types of builds?</li>
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]50 <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame]51 <li>When I use the test suite, all of the C Backend tests fail. What is
52 wrong?</li>
Reid Spencer669ed452007-07-09 08:04:31 +0000[diff] [blame]53 <li>After Subversion update, rebuilding gives the error "No rule to make
Bill Wendling174d5782007-05-29 09:35:34 +0000[diff] [blame]54 target".</li>
55 <li><a href="#llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]56 work.</a></li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]57 </ol></li>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]58
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]59 <li><a href="#felangs">Source Languages</a>
60 <ol>
61 <li><a href="#langs">What source languages are supported?</a></li>
Gordon Henriksen58366822008-02-22 20:58:29 +0000[diff] [blame^]62 <li><a href="#langirgen">I'd like to write an LLVM compiler for my language.
63 How should I interface with the LLVM middle-end optimizers and back-end
64 code generators?</a></div>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]65 <li><a href="#langhlsupp">What support is there for higher level source
66 language constructs for building a compiler?</a></li>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]67 <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
68 instruction. Help!</a></li>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]69 </ol>
70
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]71 <li><a href="#cfe">Using the GCC Front End</a>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]72 <ol>
73 <li>
74 When I compile software that uses a configure script, the configure script
75 thinks my system has all of the header files and libraries it is testing
76 for. How do I get configure to work correctly?
77 </li>
78
79 <li>
80 When I compile code using the LLVM GCC front end, it complains that it
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]81 cannot find libcrtend.a.
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]82 </li>
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]83
84 <li>
85 How can I disable all optimizations when compiling code using the LLVM GCC front end?
86 </li>
87
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]88 <li><a href="#translatec++">Can I use LLVM to convert C++ code to C code?</a></li>
89
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]90 </ol>
91 </li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]92
93 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
94 <ol>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]95 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]96 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]97 #include &lt;iostream&gt;?</a></li>
98 <li><a href="#codedce">Where did all of my code go??</a></li>
99 <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]100 </ol>
101 </li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]102</ol>
103
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]104<div class="doc_author">
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]105 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]106</div>
107
108
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]109<!-- *********************************************************************** -->
110<div class="doc_section">
111 <a name="license">License</a>
112</div>
113<!-- *********************************************************************** -->
114
115<div class="question">
116<p>Why are the LLVM source code and the front-end distributed under different
117licenses?</p>
118</div>
119
120<div class="answer">
121<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
122Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
123license, in particular one that does not compel users who distribute tools based
124on modifying the source to redistribute the modified source code as well.</p>
125</div>
126
127<div class="question">
128<p>Does the University of Illinois Open Source License really qualify as an
129"open source" license?</p>
130</div>
131
132<div class="answer">
133<p>Yes, the license is <a
134href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
135Source Initiative (OSI).</p>
136</div>
137
138<div class="question">
139<p>Can I modify LLVM source code and redistribute the modified source?</p>
140</div>
141
142<div class="answer">
143<p>Yes. The modified source distribution must retain the copyright notice and
144follow the three bulletted conditions listed in the <a
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]145href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]146</div>
147
148<div class="question">
149<p>Can I modify LLVM source code and redistribute binaries or other tools based
150on it, without redistributing the source?</p>
151</div>
152
153<div class="answer">
154<p>Yes, this is why we distribute LLVM under a less restrictive license than
155GPL, as explained in the first question above.</p>
156</div>
157
158<!-- *********************************************************************** -->
159<div class="doc_section">
160 <a name="source">Source Code</a>
161</div>
162<!-- *********************************************************************** -->
163
164<div class="question">
165<p>In what language is LLVM written?</p>
166</div>
167
168<div class="answer">
169<p>All of the LLVM tools and libraries are written in C++ with extensive use of
170the STL.</p>
171</div>
172
173<div class="question">
174<p>How portable is the LLVM source code?</p>
175</div>
176
177<div class="answer">
178<p>The LLVM source code should be portable to most modern UNIX-like operating
179systems. Most of the code is written in standard C++ with operating system
180services abstracted to a support library. The tools required to build and test
181LLVM have been ported to a plethora of platforms.</p>
182
183<p>Some porting problems may exist in the following areas:</p>
184
185<ul>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]186 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]187 compile as well on unsupported platforms.</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]188
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]189 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]190 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9)
191 will require more effort.</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]192</ul>
193
194</div>
195
196<!-- *********************************************************************** -->
197<div class="doc_section">
198 <a name="build">Build Problems</a>
199</div>
200<!-- *********************************************************************** -->
201
202<div class="question">
203<p>When I run configure, it finds the wrong C compiler.</p>
204</div>
205
206<div class="answer">
207
208<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
209<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
210for the C and C++ compiler, respectively.</p>
211
212<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
213<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
214explicitly.</p>
215
216</div>
217
218<div class="question">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]219<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
220LLVM linker from a previous build. What do I do?</p>
221</div>
222
223<div class="answer">
224<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
225if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
226it:</p>
227
228<ol>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]229 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]230 program appears first in the <tt>PATH</tt>. This may work, but may not be
231 convenient when you want them <i>first</i> in your path for other
232 work.</p></li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]233
234 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]235 correct. In a Borne compatible shell, the syntax would be:</p>
236
237<div class="doc_code">
238<pre>
239% PATH=[the path without the bad program] ./configure ...
240</pre>
241</div>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]242
243 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]244 to do its work without having to adjust your <tt>PATH</tt>
245 permanently.</p></li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]246</ol>
247
248</div>
249
250<div class="question">
251<p>When creating a dynamic library, I get a strange GLIBC error.</p>
252</div>
253
254<div class="answer">
255<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
256GCC was compiled with the --disable-shared option. To work around this, install
257your own version of GCC that has shared libraries enabled by default.</p>
258</div>
259
260<div class="question">
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]261<p>I've updated my source tree from Subversion, and now my build is trying to
262use a file/directory that doesn't exist.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]263</div>
264
265<div class="answer">
266<p>You need to re-run configure in your object directory. When new Makefiles
267are added to the source tree, they have to be copied over to the object tree in
268order to be used by the build.</p>
269</div>
270
271<div class="question">
272<p>I've modified a Makefile in my source tree, but my build tree keeps using the
273old version. What do I do?</p>
274</div>
275
276<div class="answer">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]277<p>If the Makefile already exists in your object tree, you
278can just run the following command in the top level directory of your object
279tree:</p>
280
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]281<div class="doc_code">
282<pre>% ./config.status &lt;relative path to Makefile&gt;</pre>
283</div>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]284
285<p>If the Makefile is new, you will have to modify the configure script to copy
286it over.</p>
287
288</div>
289
290<div class="question">
291<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
292</div>
293
294<div class="answer">
295
296<p>Sometimes, changes to the LLVM source code alters how the build system works.
297Changes in libtool, autoconf, or header file dependencies are especially prone
298to this sort of problem.</p>
299
300<p>The best thing to try is to remove the old files and re-build. In most
301cases, this takes care of the problem. To do this, just type <tt>make
302clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
303
304</div>
305
306<div class="question">
307<p>I've built LLVM and am testing it, but the tests freeze.</p>
308</div>
309
310<div class="answer">
311
312<p>This is most likely occurring because you built a profile or release
313(optimized) build of LLVM and have not specified the same information on the
314<tt>gmake</tt> command line.</p>
315
316<p>For example, if you built LLVM with the command:</p>
317
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]318<div class="doc_code">
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]319<pre>% gmake ENABLE_PROFILING=1</pre>
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]320</div>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]321
322<p>...then you must run the tests with the following commands:</p>
323
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]324<div class="doc_code">
325<pre>
326% cd llvm/test
327% gmake ENABLE_PROFILING=1
328</pre>
329</div>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]330
331</div>
332
333<div class="question">
334<p>Why do test results differ when I perform different types of builds?</p>
335</div>
336
337<div class="answer">
338
339<p>The LLVM test suite is dependent upon several features of the LLVM tools and
340libraries.</p>
341
342<p>First, the debugging assertions in code are not enabled in optimized or
343profiling builds. Hence, tests that used to fail may pass.</p>
344
345<p>Second, some tests may rely upon debugging options or behavior that is only
346available in the debug build. These tests will fail in an optimized or profile
347build.</p>
348
349</div>
350
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]351<div class="question">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]352<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]353</div>
354
355<div class="answer">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]356<p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]357affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]358</div>
359
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame]360<div class="question">
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]361<p>After Subversion update, rebuilding gives the error "No rule to make
362target".</p>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]363</div>
364
365<div class="answer">
366<p>If the error is of the form:</p>
367
368<div class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]369<pre>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]370gmake[2]: *** No rule to make target `/path/to/somefile', needed by
371`/path/to/another/file.d'.<br>
372Stop.
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]373</pre>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]374</div>
375
Reid Spencer669ed452007-07-09 08:04:31 +0000[diff] [blame]376<p>This may occur anytime files are moved within the Subversion repository or
377removed entirely. In this case, the best solution is to erase all
378<tt>.d</tt> files, which list dependencies for source files, and rebuild:</p>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]379
380<div class="doc_code">
381<pre>
382% cd $LLVM_OBJ_DIR
383% rm -f `find . -name \*\.d`
384% gmake
385</pre>
386</div>
387
388<p>In other cases, it may be necessary to run <tt>make clean</tt> before
389rebuilding.</p>
390</div>
391
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]392<div class="question"><p><a name="llvmc">
393The <tt>llvmc</tt> program gives me errors/doesn't work.</a></p>
Bill Wendling174d5782007-05-29 09:35:34 +0000[diff] [blame]394</div>
395
396<div class="answer">
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]397<p><tt>llvmc</tt> is experimental and isn't really supported. We suggest
398using <tt>llvm-gcc</tt> instead.</p>
Bill Wendling174d5782007-05-29 09:35:34 +0000[diff] [blame]399</div>
400
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]401<!-- *********************************************************************** -->
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]402<div class="doc_section"><a name="felangs">Source Languages</a></div>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]403
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]404<div class="question"><p>
405 <a name="langs">What source languages are supported?</a></p>
406</div>
407<div class="answer">
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]408 <p>LLVM currently has full support for C and C++ source languages. These are
409 available through a special version of GCC that LLVM calls the
410 <a href="#cfe">C Front End</a></p>
411 <p>There is an incomplete version of a Java front end available in the
Reid Spencer669ed452007-07-09 08:04:31 +0000[diff] [blame]412 <tt>java</tt> module. There is no documentation on this yet so
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]413 you'll need to download the code, compile it, and try it.</p>
Reid Spencer669ed452007-07-09 08:04:31 +0000[diff] [blame]414 <p>In the <tt>stacker</tt> module is a compiler and runtime
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]415 library for the Stacker language, a "toy" language loosely based on Forth.</p>
416 <p>The PyPy developers are working on integrating LLVM into the PyPy backend
417 so that PyPy language can translate to LLVM.</p>
418</div>
Gordon Henriksen58366822008-02-22 20:58:29 +0000[diff] [blame^]419
420<div class="question"><p><a name="langirgen">
421 I'd like to write an LLVM compiler for my language. How should I interface
422 with the LLVM middle-end optimizers and back-end code generators?
423</a></p></div>
424<div class="answer">
425 <p>Your compiler front-end will communicate with LLVM by creating a module in
426 the LLVM intermediate representation (IR) format. There are 3 major ways to
427 tackle generating LLVM IR from a front-end:</p>
428 <ul>
429 <li>
430 <strong>Call into the LLVM libraries code using your language's FFI
431 (foreign function interface).</strong>
432 <ul>
433 <li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax,
434 and .bc format</li>
435 <li><em>for:</em> enables running LLVM optimization passes without a
436 emit/parse overhead</li>
437 <li><em>for:</em> adapts well to a JIT context</li>
438 <li><em>against:</em> lots of ugly glue code to write</li>
439 </ul>
440 </li>
441 <li>
442 <strong>Emit LLVM assembly from your compiler's native language.</strong>
443 <ul>
444 <li><em>for:</em> very straightforward to get started</li>
445 <li><em>against:</em> the .ll parser is slower than the bitcode reader
446 when interfacing to the middle end</li>
447 <li><em>against:</em> you'll have to re-engineer the LLVM IR object
448 model and asm writer in your language</li>
449 <li><em>against:</em> it may be harder to track changes to the IR</li>
450 </ul>
451 </li>
452 <li>
453 <strong>Emit LLVM bitcode from your compiler's native language.</strong>
454 <ul>
455 <li><em>for:</em> can use the more-efficient bitcode reader when
456 interfacing to the middle end</li>
457 <li><em>against:</em> you'll have to re-engineer the LLVM IR object
458 model and bitcode writer in your language</li>
459 <li><em>against:</em> it may be harder to track changes to the IR</li>
460 </ul>
461 </li>
462 </ul>
463 <p>If you go with the first option, the C bindings in include/llvm-c should
464 help a lot, since most languages have strong support for interfacing with
465 C. The most common hurdle with calling C from managed code is interfacing
466 with the garbage collector. The C interface was designed to require very
467 little memory management, and so is straightforward in this regard.</p>
468</div>
469
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]470<div class="question"><p><a name="langhlsupp">
471 What support is there for a higher level source language constructs for
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]472 building a compiler?</a></p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]473</div>
474<div class="answer">
475 <p>Currently, there isn't much. LLVM supports an intermediate representation
476 which is useful for code representation but will not support the high level
477 (abstract syntax tree) representation needed by most compilers. There are no
Jeff Cohen7b8229a2006-04-26 21:03:17 +0000[diff] [blame]478 facilities for lexical nor semantic analysis. There is, however, a <i>mostly
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]479 implemented</i> configuration-driven
480 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
481 of running optimizations, linking, and executable generation.</p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]482</div>
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame]483
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]484<div class="question"><p><a name="langhlsupp">
485 I don't understand the GetElementPtr instruction. Help!</a></p>
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame]486</div>
487<div class="answer">
488 <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]489 Instruction</a>.</p>
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame]490</div>
491
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]492<!-- *********************************************************************** -->
493<div class="doc_section">
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]494 <a name="cfe">Using the GCC Front End</a>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]495</div>
496
497<div class="question">
498<p>
499When I compile software that uses a configure script, the configure script
500thinks my system has all of the header files and libraries it is testing for.
501How do I get configure to work correctly?
502</p>
503</div>
504
505<div class="answer">
506<p>
507The configure script is getting things wrong because the LLVM linker allows
508symbols to be undefined at link time (so that they can be resolved during JIT
509or translation to the C back end). That is why configure thinks your system
510"has everything."
511</p>
512<p>
513To work around this, perform the following steps:
514</p>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]515<ol>
Reid Spencer434262a2007-02-09 15:59:08 +0000[diff] [blame]516 <li>Make sure the CC and CXX environment variables contains the full path to
517 the LLVM GCC front end.</li>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]518
Reid Spencer434262a2007-02-09 15:59:08 +0000[diff] [blame]519 <li>Make sure that the regular C compiler is first in your PATH. </li>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]520
Reid Spencer434262a2007-02-09 15:59:08 +0000[diff] [blame]521 <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]522</ol>
523
524<p>
Reid Spencer434262a2007-02-09 15:59:08 +0000[diff] [blame]525This will allow the <tt>llvm-ld</tt> linker to create a native code executable
526instead of shell script that runs the JIT. Creating native code requires
527standard linkage, which in turn will allow the configure script to find out if
528code is not linking on your system because the feature isn't available on your
529system.</p>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]530</div>
531
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]532<div class="question">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]533<p>
534When I compile code using the LLVM GCC front end, it complains that it cannot
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]535find libcrtend.a.
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]536</p>
537</div>
538
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]539<div class="answer">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]540<p>
Reid Spencerf96eb572004-12-15 00:14:01 +0000[diff] [blame]541The only way this can happen is if you haven't installed the runtime library. To
542correct this, do:</p>
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]543
544<div class="doc_code">
Reid Spencerf96eb572004-12-15 00:14:01 +0000[diff] [blame]545<pre>
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]546% cd llvm/runtime
547% make clean ; make install-bytecode
Reid Spencerf96eb572004-12-15 00:14:01 +0000[diff] [blame]548</pre>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]549</div>
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]550</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]551
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]552<div class="question">
553<p>
554How can I disable all optimizations when compiling code using the LLVM GCC front end?
555</p>
556</div>
557
558<div class="answer">
559<p>
560Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
561optimizations done at the llvm level, leaving you with the truly horrible
562code that you desire.
563</p>
564</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]565
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]566
567<div class="question">
568<p>
569<a name="translatec++">Can I use LLVM to convert C++ code to C code?</a>
570</p>
571</div>
572
573<div class="answer">
574<p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
575Note that the generated C code will be very low level (all loops are lowered
576to gotos, etc) and not very pretty (comments are stripped, original source
577formatting is totally lost, variables are renamed, expressions are regrouped),
578so this may not be what you're looking for. However, this is a good way to add
579C++ support for a processor that does not otherwise have a C++ compiler.
580</p>
581
582<p>Use commands like this:</p>
583
584<ol>
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]585 <li><p>Compile your program as normal with llvm-g++:</p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]586
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]587<div class="doc_code">
588<pre>
589% llvm-g++ x.cpp -o program
590</pre>
591</div>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]592
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]593 <p>or:</p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]594
595<div class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]596<pre>
597% llvm-g++ a.cpp -c
598% llvm-g++ b.cpp -c
599% llvm-g++ a.o b.o -o program
600</pre>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]601</div>
602
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]603 <p>With llvm-gcc3, this will generate program and program.bc. The .bc
604 file is the LLVM version of the program all linked together.</p></li>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]605
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]606 <li><p>Convert the LLVM code to C code, using the LLC tool with the C
607 backend:</p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]608
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]609<div class="doc_code">
610<pre>
611% llc -march=c program.bc -o program.c
612</pre>
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]613</div></li>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]614
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]615<li><p>Finally, compile the C file:</p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]616
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]617<div class="doc_code">
618<pre>
619% cc x.c
620</pre>
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]621</div></li>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]622
623</ol>
624
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]625<p>Note that, by default, the C backend does not support exception handling. If
626you want/need it for a certain program, you can enable it by passing
627"-enable-correct-eh-support" to the llc program. The resultant code will use
628setjmp/longjmp to implement exception support that is correct but relatively
629slow.</p>
Chris Lattnerb495fb02006-08-31 04:26:31 +0000[diff] [blame]630
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]631<p>Also note: this specific sequence of commands won't work if you use a
632function defined in the C++ runtime library (or any other C++ library). To
633access an external C++ library, you must manually compile libstdc++ to LLVM
634bitcode, statically link it into your program, then use the commands above to
635convert the whole result into C code. Alternatively, you can compile the
636libraries and your application into two different chunks of C code and link
637them.</p>
Chris Lattnerb495fb02006-08-31 04:26:31 +0000[diff] [blame]638
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]639</div>
640
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]641<!-- *********************************************************************** -->
642<div class="doc_section">
643 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
644</div>
645
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]646<div class="question">
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]647<a name="iosinit"></a>
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]648<p> What is this <tt>llvm.global_ctors</tt> and
649<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
650&lt;iostream&gt;?</p>
651</div>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]652
653<div class="answer">
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]654
655<p>If you #include the &lt;iostream&gt; header into a C++ translation unit, the
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]656file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
657objects. However, C++ does not guarantee an order of initialization between
658static objects in different translation units, so if a static ctor/dtor in your
659.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]660be automatically initialized before your use.</p>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]661
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]662<p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]663STL that we use declares a static object that gets created in every translation
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]664unit that includes <tt>&lt;iostream&gt;</tt>. This object has a static
665constructor and destructor that initializes and destroys the global iostream
666objects before they could possibly be used in the file. The code that you see
667in the .ll file corresponds to the constructor and destructor registration code.
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]668</p>
669
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]670<p>If you would like to make it easier to <b>understand</b> the LLVM code
671generated by the compiler in the demo page, consider using <tt>printf()</tt>
672instead of <tt>iostream</tt>s to print values.</p>
673
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]674</div>
675
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]676<!--=========================================================================-->
677
678<div class="question"><p>
679<a name="codedce"></a>
680Where did all of my code go??
681</p></div>
682
683<div class="answer">
684<p>
685If you are using the LLVM demo page, you may often wonder what happened to all
686of the code that you typed in. Remember that the demo script is running the
687code through the LLVM optimizers, so if your code doesn't actually do anything
688useful, it might all be deleted.
689</p>
690
691<p>
692To prevent this, make sure that the code is actually needed. For example, if
693you are computing some expression, return the value from the function instead of
694leaving it in a local variable. If you really want to constrain the optimizer,
695you can read from and assign to <tt>volatile</tt> global variables.
696</p>
697</div>
698
699<!--=========================================================================-->
700
701<div class="question"><p>
702<a name="undef"></a>
703<p>What is this "<tt>undef</tt>" thing that shows up in my code?
704</p></div>
705
706<div class="answer">
707<p>
708<a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
709a value that is not defined. You can get these if you do not initialize a
710variable before you use it. For example, the C function:</p>
711
712<div class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]713<pre>
714int X() { int i; return i; }
715</pre>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]716</div>
717
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]718<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has
719a value specified for it.</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]720</div>
721
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]722<!-- *********************************************************************** -->
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]723
724<hr>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]725<address>
726 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
727 src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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730
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]731 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]732 Last modified: $Date$
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]733</address>
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John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]735</body>
736</html>